1) Field of the Invention
This invention relates to a method for improving the performance of copper foils and copper-based alloy foils having a coating on the surface of the foil to resist oxidation and to the foils produced by such a method. More specifically, the invention relates to a specific electrolyte solution for depositing an improved oxidation resistant coating on the surface of copper and copper-based alloy foils.
2) State of the Art
Copper foil is widely used in the electronic industry in the production of many electrical devices and circuits, e.g., printed circuit boards, integrated circuits, electrical contact surfaces and the like.
One of the problems which faces manufacturers of both electrolytic and wrought copper and copper alloy materials for electrical applications is the tarnishing of the materials during storage and shipment. This tarnishing generally results from the exposure of the copper to normal atmospheric conditions. This tarnishing is aesthetically unpleasant as well as a potential source of serious problems during the manufacture of many products such as printed circuits. For example, tarnishing of copper foil prior to its lamination to a dielectric substrate can affect both the bond strength between the foil and the substrate material and the etching characteristics of the resultant laminate.
In the past, tarnish resistance has been imparted to copper and copper base alloy materials by immersion of the copper material in an electrolyte containing chromate ions. U.S. Pat. No. 3,625,844 to McKean describes a method of effecting an antitarnish treatment of copper foil involving the electrolytic treatment of copper foil in an aqueous electrolyte under critical conditions of hexavalent chromium ion concentration, cathode current density, and treatment time. U.S. Pat. No. 3,853,716 to Yates et al. discusses the McKean process and points out that it is not a completely satisfactory antitarnishing technique because a build-up of copper and chromium cations in the electrolyte bath interfere with the effectiveness of the antitarnish treatment. Yates et al. attempt to overcome this problem by rendering the copper material cathodic as it passes through an aqueous electrolyte containing hexavalent chromium ion-containing anions while the aqueous electrolyte is of sufficient alkalinity to cause precipitation of copper and chromium cations. U.S. Pat. Nos. 4,131,517 to Mitsuo et al. and 4,387,006 to Kajiware et al. illustrate still other chromate containing treatments for suppressing time-dependent changes in color tone during storage. Still other antitarnish treatment techniques are illustrated in U.K. published patent applications 2,030,176A and 2,073,779A.
The purpose of the above-discussed antitarnish layer is to prevent oxidation or tarnishing of the copper foil which, as previously pointed out, is not only unpleasant in appearance, but also is an indication of a reacted or nonpure copper foil surface. In addition to the techniques referred to above, the layer can be applied through a dip or electroplating process, and the material used can be organic, inorganic or a combination of the two.
As laminating and processing conditions have changed over the years, so too have the demands on the stabilizer, i.e., antitarnish treatment, performance. Presently, laminators require that the foil show no discoloration after lamination and subsequent processing. In the United States, this means that the foil must withstand temperatures of 180.degree.-190.degree. C. for one hour, however, as laminators begin to use higher Tg resin systems and continuous lamination processes, the foil will have to withstand 250.degree. C. for 1/2 hour.
Present stabilization schemes involve the deposition of a zinc layer onto the copper foil followed by a chromate conversion reaction to form a zinc-chromate layer on the foil. The layer is stable but often nonuniform. Nonuniformity is manifested on a macro-scale by the appearance of bands and streaks, and on the micro-scale by the appearance of small spots. As a means of increasing oxidation protection and decreasing the nonuniformities observed in the stabilization layer, a silane or similar material can also be applied to the foil surface on top of the zinc-chromate layer.
It is known in the art that a chromium-zinc compound forms a satisfactory anti-tarnish coating for copper and copper base alloys. One such commercial coating has the composition 10 atomic % Zn; 5% Cr; 37% O; 46% C and 2% Cu. The coating is readily removed with a 4% HCl solution.
Examples of such stabilization schemes and treatment processes are disclosed in U.S. Pat. Nos. 4,952,285, 5,022,968, 5,057,193 and 5,098,796. Also U.S. Pat. No. 4,456,508 discloses multilayer treatment of copper foil for the primary purpose of improving bond strength of the foil with a base material.